While rolled steel products can be fabricated with a variety of cross sectional shapes for concreting, i.e. embedding or sheathing in concrete, it is of particular interest to provide rolled steel stock in rod or bar shape which can be incorporated in concrete vessels as reinforcing rings or the like as will be discussed in greater detail below. Such stock, adapted to be sheathed, embedded or encased in concrete will be referred to hereinafter as concrete bar or as rolled stock or by terms of similar significance.
It is known to provide concrete bar having a yield strength of the order of 400 N/mm.sup.2 but which only have a low toughness. Thus their transition temperature for energy absorption for the Charpy V impact test at 35 J/cm.sup.2 is of the order of +20.degree. C. It follows that such rolled products do not possess any significant ductility at lower temperatures.
However, it is of increasing interest to be able to form reinforced concrete vessels which are capable of operating at temperatures well below this transition temperature and ambient temperature. For example, it is desirable to fabricate concrete vessels for containing liquefied natural gas or other liquefied gases at temperatures which may be as low as -196.degree. C. and hence to provide concrete reinforcing steel stock which has a high energy absorption as low as this temperature and, more particularly, has a transition temperature for the energy absorption for the Charpy V impact test at 35 J/cm.sup.2 of the order of -196.degree. C.
Low-cost concrete bar stock was not previously available.
Consequently, in the cryogenic field and especially the field of low temperature gas storage, it has been necessary to utilize large quantities of expensive reinforcing steels having a good ductibility at low temperature. The reinforcing steels could be embedded in concrete surrounding a storage tank for low temperature liquefied gas in the same manner as concrete containment vessels were applied in the nuclear reactor field.
In order to be suitable for use as a reinforcement for a concrete containment around a liquefied gas reservoir which is subjected to temperatures between -50.degree. C. and -196.degree. C., the concrete bar must have a high notch impact toughness throughout its cross section, and be easily welded, thereby employing a carbon level less than 0.2% by weight. When conventional concrete bar containing 0.16 to 0.2% carbon are subjected to cold twisting, it is frequently noted that they may have a sufficient yield strength but an insufficient toughness at the lower temperatures.
When attempts are made to have carbon contents lower than 0.2% and the bar are rapidly cooled at their surface upon leaving the rolling line, an autotempering occurs which yields weldable and tough bar but with a notch impact toughness such that the transition temperature of the Charpy V 35 J/cm.sup.2 test is about -50.degree. C.
It is possible to provide concrete bar utilizing a steel alloy with 9% nickel and which is subjected to a double normalizing followed by a tempering or quenching followed by a tempering to achieve an energy absorption of 35 J/cm.sup.2 for the Charpy V impact test with a transition temperature of -196.degree. C.
However, such additional heat treatments are time consuming and expensive, as is the resulting alloy.